The aim of the proposed project is to obtain basic information on the site and mechanism of synthesis of the major surface active components of pulmonary surfactant - the phospholipids dipalmityl-phosphatidylcholine (PC) and phosphatidylglyserol (PG) - both in the adult rabbit lung and during fetal development. In particular the role of the alveolar type II cell and lamellar body fraction in the synthesis of PC, especially by deacylation and reacylation of preformed PC, and PG will be established. The lipid composition of fetal rabbit lung wash and whole lung as well as the activities of enzymes of phospholipid synthesis in the fetal rabbit lung during development will be measured in order to determine the normal developmental profile of events associated with surfactant production. The effect of administration of cortisol and thyroxine directly to the rabbit fetus on the above biochemical parameters will then be assessed in order to determine the mechanism of action of these hormones in the acceleration of fetal lung maturation. It will be determined whether stimulation of enzyme activity by hormones, such as stimulation of the activity of glycerolphosphate phosphatidyltransferase by cortisol administration to fetal rabbits, is due to new protein synthesis or merely stimulation of catalytic activity. These studies are of clinical importance since immaturity of the lung with insufficient surfactant is believed to be the cause of the respiratory distress syndrome of the newborn and information on lung maturation and acceleration is essential for its prevention and/or control.